Cook pasta in a large pot of boiling salted water. Once it reaches al dente, drain and set aside while you finish the sauce.

Meanwhile, heat olive oil in a heavy-bottomed Dutch oven over medium heat. Add garlic and season with salt. Cook, stirring occasionally, until garlic is lightly browned and toasted around the edges, about 2 minutes. Add red-pepper flakes, if using.

Add tomatoes and then fill the empty can about 3/4 of the way up with water. Swirl the can to loosen the tomatoey bits left behind and add that to the pot. Season with salt, bring to a simmer and cook, stirring occasionally, until sauce has thickened slightly (it should still look quite brothy), 10 to 15 minutes.

Add the mussels or clams to the sauce, if using, along with the fish, and season again with salt. Let the seafood settle into the brothy tomato sauce and gently swirl the pot, encouraging the seafood to cook evenly. Cook a minute or two, then add the shrimp, swirling the skillet again. Cook until all the seafood is just cooked through and the mussels or clams have opened, another 3 to 5 minutes.

Add the pasta and very gently toss to coat, cooking another 1 to 2 minutes, just to let the flavors meld. Add olives and remove from heat.

Divide among bowls, top with parsley and drizzle with olive oil before serving.